Under-Display Optical Proximity Sensor With Silicon Nanowires

Abstract

This paper presents a novel optical proximity sensor behind a display panel based on silicon nanowires. Since silicon nanowires have high photoresponsivity, they can sense the distance even when placed under the display, without using a large current for driver operation. In addition, the dark leakage current generated by silicon nanowires can be removed through the read-out integrated circuit (ROIC) in the proposed optical proximity sensor. The ROIC has a resistive-feedback transimpedance amplifier (TIA) to provide a constant voltage to silicon nanowires and to convert the photocurrent of the silicon nanowires. The auto-zeroing technique reduces the ROIC's offset and 1/f noise. The bias sampling is applied to a 12-bit single-slope analog-digital converter to reduce noise generated in the bias circuit of the ramp generator. The digital back-end cancels the offset due to dark leakage current through the correlated double sampling technique. The proximity sensor is fabricated in a 0.18 mu m CMOS process and the ROIC occupies an area of 0.18 mm(2). It draws an average current of 139.4 mu A from a 3 V supply, and 60.2 mu A from a 1.8 V supply when the conversion time is set to 50 ms, and the current of the vertical cavity surface emitting laser (VCSEL) driver is set to 12 mA. The proximity sensor is tested under a display panel with a light transmission of 6 %. It has an input-referred root mean square noise of 323 pA. The VCSEL driving current can be varied to 7 mA, 10 mA, and 12 mA, and the optical proximity sensor can detect an object that is closer than 40 mm in all three cases.